This invention relates to a fast heating cathode (FHC).
A typical example of an application for a FHC is in small Travelling Wave Tubes (TWT). TWT devices require an electron gun to supply a stream of high energy electrons through an amplifying structure. The source of these electrons is normally a heated cathode, with the electron emission being a result of thermionic emission. The electrons emitted are accelerated through the amplifying section of the TWT by the application of a high voltage differential (typically 10–20 kV) between the cathode and the collector within the TWT.
Considerable effort is expended to ensure that the electron emission from the cathode surface is uniform across the emitting region and that the cathode remains at the ideal operating temperature. As a result of these requirements, the majority of cathodes used within TWT type devices require a period of time to temperature stabilise. For devices where the application may demand a more immediate use than is permitted by this stabilisation period, the device must be maintained in the “switched-on” mode.
A device which is maintained in the “switched-on” mode to avoid the lengthy stabilisation period also has disadvantages. In particular, the device needs a constant power supply and is a continual power drain. In addition, as the cathode life is finite, the total operation lifetime of the device is severely shortened, and failure may occur at an inconvenient moment.
There are two alternatives to these conventional hot cathodes. These are (a) “cold cathodes” where the work function of the material is such that electrons can move freely from the material into space at normal environmental temperatures, and (b) some form of fast heating cathode (FHC). Cold cathodes cannot at this time provide a suitable device for the applications mentioned above.
Fast heating cathodes under current development are based on conventional technologies, but using enhanced engineering designs. Typically, they use a tungsten or tantalum wire filament acting as the electron emitter, heated by a heater which is electrically isolated to avoid voltage drops along the emitter itself. Most developments are based on modifications to the method of applying the heat rapidly and uniformly, including techniques as diverse as lasers and electron beam guns.